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How To Rebuild Your RC Engine - Diagnosis and Teardown Part One

Article by Kenny McCormick

Engines are simple, and when well cared for, they last and last. But nothing lasts forever. Perhaps you've got 25 gallons on your engine. Perhaps it sucked in a june bug. Perhaps one of the circlips holding the wrist pin in broke.

Either way, you'll eventually run into an engine that just flat won't run anymore, and if it has a fair bit of sentimental value and/or is expensive to replace, rebuilding it is a viable option. Here's how to do that.

These tutorials on RC engine dissasembly and reassembly apply to pretty much any engine. Whether you race cars, trucks, or fly rc planes and helis, you will definitely get something out of this rebuild series. The same prinicples apply no matter what type of RCing you do.

First things first: You need a clean area to work. Dirt, any dirt, will ruin an engine. The engine in this article is a victim of a dirty reassembly, matter of fact. The pictures you see are of an engine that had foreign particles floating around inside it while it was running. Once you have a clean area to work with, you'll need tools. There won't be much needed, an allen key to fit the head and backplate bolts, a small wrench or nutdriver for the carb retention bolt, and a philips and common screwdriver. You will also need a plastic or wooden dowel of some sort. A mechanical pencil is a good example.

This is a good setup. All tools needed, the engine, clean work area. Not pictured: new parts

With your work area situated, remove the engine from the vehicle, then remove the various accessories. Leave the carb attached for now. The engine used in this article is a .45CID aircraft two stroke, this was chosen because the parts are larger and thus easier for my camera to take decent closeups of. Car engines are built the same inside, the main difference being some car engines have a start shaft in the backplate as well as a pin of some sort to connect said start shaft to the crankshaft.

The first step is to clean the engine. Mine was already clean when I received it, but yours most likely won't be. Especially if it's just simply worn out. So clean it. You don't need it looking brand new, but you need all the dust, dirt and grit that could get into the engine removed. With the engine clean, locate the proper allen key or screw driver that fits the head bolts and remove them. THEY WILL BE TIGHT!... or at least, they had better be. Break them loose in a criss-cross pattern and once they're loose, spin 'em out however you see fit. With the head bolts removed, carefully pop the head off. Car engines have large heat-sink heads that should make this easy, just gently twist it and it should pop off. Airplane engines will put up more of a fight if they've been on there a while, but you can get away with carefully prying the head up and off. Take care to not nick the sleeve while doing so. When the head comes off, there will be a copper or aluminum washer, sometimes more than one, either stuck to the head or the sleeve. DO NOT LOSE THIS! It is the head gasket, your engine will not run right without it. May not run at all. Oh, and when you get the head off, thread the head bolts back into the block four or five turns by hand just to ensure you don't lose them.

Matching pitting on piston, more not good. Also, thread the head bolts back into the block to keep them from getting lost.

Once you remove the head, turn it over and inspect it. It should be free of pitting, scratches, dirt, and mechanical damage, and it should have a golden brown appearance. If it is pitted like mine the engine failed either because of a lean run causing detonation or mechanical damage caused by debris bouncing around. Mine is the latter. Inspect the top of the piston too, it should be clean of dirt, not pitted, and golden-brown. Again, if it's pitted, you had detonation and/or debris floating around inside the engine.

Rotate the cylinder to bottom dead center and look at the cylinder walls. In a taper-bored (ABC and ABN) engine it should be shiny and smooth. There should also be a thin film of oil on it. In the picture above you can see deep gouges, this is caused by a piece of wayward metal floating around inside the engine, which I later found in one of the transfer ports. If your sleeve is dull, but still smooth, you likely got excellent life out of the engine and simply wore it out. Normal old-engine failure. If your sleeve looks like mine, or if the chrome/nickel plating is peeling and flaking off, you had some sort of outside factors destroy the engine before its time that need to be rectified before you run the rebuilt engine.

Look at the bottom of the cylinder. That scratch is why this engine is not in running condition.

Why are we so interested in why the engine failed? It's pretty important we know why it failed so that it doesn't blow up again shortly after we get it running again. Rebuilds are expensive, after all, we don't want to do it twice in a row.

The Grand Canyon here matches up with the scratches in the sleeve. It should look like the image below:
This is what the piston is supposed to look like.

Now that we've figured out why the engine failed, it's time to start removing parts. The engine should be sitting at bottom dead center right now. Remove the backplate the same way you removed the head. On some engines, like mine, there is a rubber O-ring. Others have a small fiber gasket. If you have an O-ring, and you're careful, you should be OK to re-use it. If you have a fiber gasket you must replace it regardless of how intact it comes off. If your engine failed because of old age, look into replacing the O-ring as well, time does degrade the rubber and it may not seal after reassembly. Once the backplate is off, note any flat cut into it. My engine has a flat in it which points towards the top of the engine, and must be reinstalled this way or the engine simply will not turn over. Most engines are made this way.

Replace the fiber gasket if you have one! Replacing the o-ring doesn't hurt either.
Piston clearance flats go towards the piston!

With the backplate off, we should be able to see inside the crankcase. There should be a lot of oil residue in here. Some of it will still be oil, some of it will be more of a golden-brown varnish on the parts. If you have dirt or scratches inside the crankcase your engine failed because of dirt ingestion, or perhaps a failed main bearing. If you find rust back here as well as scratches, the failure was due to the engine corroding and then being run without being cleaned out properly. If there's no damage to the crankcase in any way, but you still have debris damage to the piston and cylinder, look towards either something left in the cylinder during assembly or a failed circlip on the wristpin.

They never slide out easily. Note the head bolts, they'll keep the sleeve from falling out until you're ready for it to do so
DO NOT DO THIS WITH METAL OBJECTS! Plastic, rubber, wood, these are accepable. If it's metal it is not."